Ask your vet why - even though you have been on three full rounds of targeted antibiotics - some of your snakes are still sick.
If it's a bacterial infection and it was properly cultured, there is no reason why it should be able to survive three rounds of antibiotics that it was cultured sensitive to.
Viral infections can travel from rack to rack. Some people's entire collections have been wiped out by a viral infection, regardless of whether or not they were in the same rack or in another room.